The patent literature discloses a plasma generator for generating air bubbles in a channel that includes a narrow portion, further causing plasma to generate in the air bubbles, and measuring the resulting light emission. In patent literature 1, plasma in the narrow portion of the channel is measured, and in patent literature 2, plasma in a region other than the narrow portion is measured. However, in each of the described devices, there is a problem because the reproducibility of the plasma light emission is low.